1. Field of the Invention
This invention is concerned with a fluidized catalytic cracking process wherein coked deactivated catalyst is subject to high temperature stripping to control the carbon level on spent catalyst. More particularly, the concept employs a high temperature stripper to control the carbon level on the spent catalyst, followed by catalyst cooling to control the temperature of the catalyst to regeneration.
2. Description of the Prior Art
The field of catalytic cracking has undergone progressive development since 1940. The trend of development of the fluid catalytic cracking process has been to all riser cracking, use of zeolite-containing catalysts and heat balanced operation.
Other major trends in fluid catalytic cracking processing have been modifications to the process to permit it to accomodate a wider range of feedstocks, in particular, feedstocks that contain more metals and sulfur than had previously been permitted in the feed to a fluid catalytic cracking unit.
Along with the development of process modifications and catalysts, which could accomodate these heavier, dirtier feeds, there has been a growing concern about the amount of sulfur contained in the feed that ends up as SO.sub.x in the regenerator flue gas. Higher sulfur levels in the feed, combined with a more complete regeneration of the catalyst in the fluid catalytic cracking generator tends to increase the amount of SO.sub.x contained in the regenerator flue gas. Some attempts have been made to minimize the amount of SO.sub.x discharged to the atmosphere through the flue gas by providing agents to react with the SO.sub.x in the flue gas. These agents pass along with the regenerated catalyst back to the fluid catalytic cracking reactor, and then the reducing atmosphere releases the sulfur compounds as H.sub.2 S. Suitable agents for this purpose have been described in U.S. Pat. Nos. 4,071,436 and 3,834,031. Use of a cerium oxide agent for this purpose is shown in U.S. Pat. No. 4,001,375.
Unfortunately, the conditions in most fluid catalytic cracking regenerators are not the best for SO.sub.x adsorption. The high temperatures encountered in modern fluid catalytic cracking regenerators (up to 1600.degree. F.) tend to discourage SO.sub.x adsorption. One approach to overcome the problem of SO.sub.x in flue gas is to pass catalyst from a fluid catalytic cracking reactor to a long residence time steam stripper. After the long residence time steam stripping, the catalyst passes to the regenerator, as disclosed by U.S. Pat. No. 4,481,103 to Krambeck et al, which is incorporated herein by reference. However, this process preferably steam strips spent catalyst at 932.degree. to 1022.degree. F. (500.degree.-550.degree. C.), which is not sufficient to remove some undesirable sulfur- or hydrogen-containing components. Furthermore, catalyst passing from a fluid catalytic cracking stripper to a fluid catalytic cracking regenerator contains hydrogen-containing components, such as coke, adhering thereto. This causes hydrothermal degradation when the hydrogen reacts with oxygen in the regenerator to form water.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, the flue gas from both stages of regeneration contains SO.sub.x which is difficult to clean.
Another need of the prior art is to provide improved means for controlling fluid catalytic cracking regeneration temperature. Improved regenerator temperature control is desirable, because regenerator temperatures above 1600.degree. F. (871.degree. C.) can deactivate fluid cracking catalyst. Typically, the temperature is controlled by adjusting the CO/CO.sub.2 ratio produced in the regenerator. This control works on the principle that production of CO produces less heat than production of CO.sub.2. However, in some cases, this control is insufficient.
It would be desirable to separate hydrogen from catalyst to eliminate hydrothermal degradation. It would be further advantageous to remove sulfur-containing compounds prior to regeneration to prevent SO.sub.x from passing into the regenerator flue gas. Also, it would be advantageous to better control regenerator temperature.
U.S. Pat. No. 4,353,812 to Lomas et al discloses cooling catalyst from a regenerator by passing it through the shell side of a heat-exchanger with a cooling medium through the tube side. The cooled catalyst is recycled to the regeneration zone. This process is disadvantageous, in that it does not control the temperature of catalyst from the reactor to the regenerator.
The prior art also includes fluid catalytic cracking processes which utilize dense or dilute phase regenerated fluid catalyst heat removal zones or heat-exchangers that are remote from, and external to, the regenerator vessel to cool hot regenerated catalyst for return to the regenerator. Examples of such processes are found in U.S. Pat. Nos. 2,970,117 to Harper; 2,873,175 to Owens; 2,862,798 to McKinney; 2,596,748 to Watson et al; 2,515,156 to Jahnig et al; 2,492,948 to Berger; and 2,506,123 to Watson. The processes disclosed in these patents have the disadvantages that the regenerator operating temperature is affected with the temperature of catalyst from the stripper to the regenerator.